There has been increasing worldwide interest in the field of technical textile materials. Within this context the use of membranes for industrial separation processes has developed, and they can now compete effectively with conventional processes in terms of energy and capital costs. Membranes for gas separation have developed significantly in the last twenty years; however, there is still a need for high-temperature and chemically resistant membranes that exhibit good selectivity and gas permeability. In spite of the developments in gas separation membranes, only a few types of hollow-fibre membranes are still commercially available. Our study examines the fundamental properties of polyetherketone (PEK, a thermally stable and chemically resistant polymer) membranes prepared using concentrated sulphuric acid (98% H2SO4) as a solvent and dilute sulphuric acid (30%-60% H2SO4) as a non-solvent. Other non-solvents included acetic acid, ethanol, methanol, glycerol, and water. The concentration of the polymer-casting solutions was between 15% and 20%. The membrane structure was examined using SEM, and the gas separation properties were measured using a lab-scale test rig. The results show that formation and control of membrane structure are complicated, and that many preparation parameters affect membrane morphology and performance. Polymer concentration is a particularly important parameter. At each individual polymer concentration, the precipitant plays a crucial role, and has a determining influence on membrane structure. Membranes cast using 30-40% glycerol and 50-60% H2SO4 or 70-90% acetic acid as precipitants possessed sponge-type structures, and as such have an acceptable permeation rate. However, membranes cast into water display finger-like structures even at a low coagulation temperature of 3?C, and also exhibit lower permeation rates. It has also been shown that precipitated structures of PEK membranes are highly dependent upon the heat of mixing of the solvent with non-solvent, and that a reduction in this heat of mixing leads to sponge-like structures that are preferential for gas separation membranes.